The p'shat in the gemorah is that since he is a Talmud Chacham and has the 'shmirah' of his Torah he doesn't need to lein Krias Shema before going to bed. Although we see in this how Rashi describes a Talmud Chacham....שרגיל במשנתו לחזור על גרסתו תמיד

Update (July 3rd 2012): See a response to my post at the Parsha Blog, Here.